This project proposes a set of lab and field experiments to further the fundamental understanding of biogeochemical processes governing the behavior and transport of inorganic and organic Arsenic (As) species as well as contribute to the design and management of remediation activities of sites contaminated with As. We have chosen the Vineland Chemical site in southern New Jersey as our primary field site because it offers a number of research opportunities. One primary focus will be on experiments that could help optimize pump and treat operations. These studies would include plume capture efficiency assessments through measurements of tracers of groundwater flow velocities and directions. They would also involve laboratory and field experiments to investigate whether manipulations of aquifer chemistry could enhance remediation of groundwater As, primarily through accelerating As mobilization from aquifer solids, thereby enabling pump and treat operations to decrease subsurface As inventories more rapidly. We will also collaborate with Project 7 on investigating the feasibility of using permeable reactive barriers of zero valent iron as an additional remediation strategy to the current pump and treat system. Other field experiments will investigate fundamental transport and fate issues of As in two offsite areas highly contaminated with As, specifically the Blackwater Branch and Union Lake. We will further investigate As cycling in bottom sediments of both the stream (which runs adjacent to the Superfund site) and of Union Lake. Our prior work has shown that the sediments of these two surface water bodies have dramatically different As chemistries, with fine-grained sediments of the stream having As controlled largely by sulfur chemistry while As in the lake sediments appears to be primarily controlled by iron chemistry. Additional sediment-porewater research will be done in the Blackwater Branch to investigate whether As behavior changes as a function of streamflow/groundwater discharge rates and to investigate As porewater-sediment interactions in sandy sediments which represent >80% of the stream bottom. The role of storm events in transporting As in the Blackwater Branch will also be investigated. Finally, research in Union Lake will investigate the magnitude of As release from sediments during episodes of anoxia in bottom waters.